1. Field of the Invention
The present invention relates to drill pipe stabilizers or torque reducers used with drill strings. Additionally, the invention relates to devices used to protect wellbore casing.
2. Description of Related Art
During drilling operations, contact between the drill string and wellbore casing results in undesirable friction and off-bottom torque forces, which are applied against the drill string. This torque reduces drilling effectiveness and may result in damage to the drill string if the torque is particularly high or sustained for a long period of time. When a drill string is disposed into a deviated well, such as during an extended reach drilling operation, portions of the drill pipe tend to come into contact with the casing at points where the wellbore deviates. At these points of contact, the rotating drill string causes wear and damage to occur on the drill string. Rotation of the drill string also tends to wear the casing thus reducing wall thickness and casing strength. Ultimately, this type of wear can cause holes to develop in the casing. In some cases, drilling operations need to be stopped for emplacement of casing patches over the holes.
Devices known as drill pipe stabilizers or torque reducers are used during drilling in wellbores to protect the well casing. These devices are used to surround portions of the drill pipe and are intended to protect the drill pipe from the damaging torques and wear that can occur from rotation of the drill pipe against the casing. Unfortunately, most known stabilizers are either difficult to attach and to remove from a drill string, or they result in often unwanted additional length to the drill string. Further, conventional stabilizers often fail to adequately protect the drill string. Because most current designs affix the stabilizer directly to the drill pipe itself, wear occurs on the drill pipe during drilling operations.
Additional length results from the use of separate stabilizer or torque reduction subs that are incorporated into the tool string by separating the string at a tool joint, inserting the sub and reassembling the drill string. These subs typically consist of a mandrel having a surrounding metal sleeve. The sleeve is free to rotate about the sub's mandrel using sealed bearings which are disposed between the sleeve and the mandrel. An example of this type of sub is the Security DBS Drill String Torque Reduction Sub.
One device which became available in the early 1990's is referred to as the nonrotating drill pipe/casing protector (or NDPP). The NDPP consists of a rubber sleeve which is held in place axially on the drill pipe with aluminum thrust collars. The collars are secured to the drill pipe by bolts which clamp it against the drill pipe. One major drawback of this tool is the potential for it to strip, leaving the rubber element downhole with consequential fishing or directional drilling problems.
Smith International also currently markets a non-rotating sleeve-type stabilizer sub. The sub body includes a wear mandrel having a hardened wear surface. An elastomeric sleeve surrounds the wear mandrel. The sleeve is made of a single cylindrical piece and presents either four or six radially outwardly-extending ribs.